This invention relates to the AC etching of aluminum capacitor foil in a hydrochloric acid-based electrolyte containing at least one additive and additionally 200 to 500 ppm sulfate to permit the etching of 3.0 to 5.0 mil thick foil for higher voltage use than with the same electrolyte without sulfate.
AC etching has been used to produce aluminum articles such as lithographic plates and capacitor foil. It also has been used to electropolish aluminum articles prior to metal plating.
The prior art has discussed the difficulties in obtaining an even or uniform etch structure and has overcome these difficulties in a variety of ways, e.g. interrupting the etch process to apply protective coatings, carrying out the etch process in stages of differing degrees of aggressiveness, and using additives in the electrolyte bath to control pit size or to increase mechanical strength in the foil.
Another problem has been to prevent the precipitation of aluminum hydroxide, formed during etching, on or into the etched surfaces.
The resolution of these problems has led to prior art processes in which the etch conditions are carefully controlled to provide the desired increase in surface area and, particularly for capacitor foil, with little change in mechanical strength. Such a process is described by M. R. Arora, E. J. Paquette, and A. B. McPherson in U.S. Pat. No. 4,332,652 issued June 1, 1982 which utilizes phosphoric and nitric acids in the hydrochloric acid etch bath. Another such process is described by M. R. Arora in copending U.S. Ser. No. 321,973, filed Nov. 16, 1981, now U.S. Pat. No. 4,376,686 issued Mar. 15, 1983, which utilizes nitric and tartaric acids in the hydrochloric acid etch bath.